1. Field of the Invention
The present invention relates to a data communication apparatus and a communication data control method. More particularly, it relates to, for example, a data communication apparatus and a communication data control method in which voice information obtained through a wireless microphone or the like is transmitted by means of wireless communication.
2. Description of the Related Art
To date, wireless-microphone systems have been used as voice collection and amplification systems. Among these wireless-microphone systems, there are types of wireless-microphone systems each of which includes a voice collecting microphone configured to be separate from a transmitter, and allows various types of microphones to be attached to the transmitter in accordance with a user's demands. With respect to the microphones, there are various types, such as a compact microphone, which is hardly visible even while being used, and a highly directional microphone, which is used for collecting voices coming from far away. More specifically, for example, a compact microphone which can be attached to a user's clothing, i.e., a lavalier microphone, or a bar-shaped microphone which has a sharp directional characteristic, i.e., a shotgun microphone, can be used. Among these types of microphones, there are types of microphones each incorporating therein an amplifier for amplifying voice signals using an element, for example, an active element such as an FET or a transistor.
In FIG. 1, an example of a configuration of a communication system constituted by a microphone 10, a transmitter 20 and a receiver 30 is shown. The microphone 10 includes a vibration plate 11 configured to vibrate in accordance with acoustic waves originating from users' voices and the like, and an FET 12 configured to amplify electric signals generated on the basis of the vibrations, and, outputs signals amplified by the FET 12 to the transmitter 20. The microphone 10 is supplied with electric power through, for example, the transmitter 20.
Voice signals outputted from the microphone 10 are inputted to the transmitter 20. The transmitter 20 performs processes of analog-to-digital conversion, digital modulation, high-frequency amplification and the like, and thereby, produces transmission signals to be transmitted to the receiver 30. The transmission signals are transmitted to the receiver 30 via an antenna 21 by means of wireless communication.
The receiver 30 receives communication data from the transmitter 20 via an antenna 31, further, performs a process of demodulating in order to extract the voice signals from received signals, and then, outputs the resultant signals to, for example, a speaker, a recording device or the like, which are not illustrated in FIG. 1. For example, the demodulated voice signals are outputted through the speaker, or alternatively, are subjected to processing for recording thereof into a recording medium in the recording device.
A configuration of the transmitter 20 and processes performed by the transmitter 20 in a system in which the transmitter 20 performs wireless transmission subsequent to digital modulation in such a manner as described above will be hereinafter described with reference to FIG. 2. FIG. 2 is an example of a configuration of the transmitter 20 configured to produce transmission data from electric signals inputted thereto, which are generated on the basis of the voice signals collected by the microphone 10. The transmitter 20 includes an amplifier 25, an analog-to-digital convertor 26, a packet producer 27, a digital modulator 28, a high-frequency amplifier 29 and an antenna 21.
Electric signals generated on the basis of voice signals collected by the microphone 10 are amplified by the amplifier 25 included in the transmitter 20, and subsequently, are converted to digital data by the analog-to-digital converter 26. The packet producer 27 produces packet data including this digital data. In the packet data, pieces of synchronization data, which are used for data synchronization, are provided.
Transmitting packet data has packets each including a piece of synchronization data and a block of voice data. An example of a configuration of the transmitting packet data is shown in FIG. 3. As shown in FIG. 3, the transmitting packet data forms a pattern of data in which packets each including a piece of synchronization data 51 and a block of voice data 52 are repeatedly arranged.
The digital modulator 28 performs a process of digital modulation with respect to the packet data, and subsequently, the high-frequency amplifier 29 superimposes packet data resulting from performing the process of digital modulation on a carrier signal functioning as a carrier wave of high frequency, further, performs a process of high-frequency amplification with respect to the resultant signals, and then, outputs the transmission signals generated thereby via the antenna 21.
In this manner, the transmitter 20 performs processes of modulation, high-frequency amplification and the like with respect to signals inputted from the microphone, and then, radiates wireless transmission signals. However, for example, a user uses the microphone 10 and the transmitter 20 under the condition that they are attached to the user's body. In this case, as a result, the microphone 10 and the antenna 21, which is configured to transmit data outputted from the transmitter 20, are located close to each other.
In such a case that the microphone 10 and the transmitter 20 are located close to each other, an active element, such as the FET 12 or the like, is likely to detect radio waves radiating from the antenna 21 of the transmitter 20. The detected signals are added to voice signals as noise signals which are different from the voices targeted for collection such as speech voices originating from a user, and as a result, are transmitted to a data converting section of the transmitter 20.
The transmitter 20 produces packets including the blocks of voice data 52 resulting from conversion of input signals from the microphone 10 as they are, that is, input signals, which include the voice signals to which the noise signals are added, as they are. Theses packets including the noise signals therein are transmitted to the receiver 30 as radio communication signals.
The receiver 30 receives transmission data from the transmitter 20, further, performs demodulation of voice data, and thereby, extracts voice signals. The extracted data is made up of voice signals including noise signals. Reproduction of voice signals in accordance with processes performed by the receiver 30 results in reproduction of voice signals to which noise signals are added, and further, depending on the utilization purpose of the system, the quality of the reproduced signals is likely to be unacceptable with respective to the desired quality of reproduction.
Components of the noise signals are associated with transmission cycles inherent in a string of data to be transmitted. In the case where pieces of data each having the same pattern are transmitted at intervals of a constant period, the pieces of data are reproduced as a sine wave having a frequency which is the reciprocal of the constant period, and in the case where a string of data having a random pattern with no transmission cycle is transmitted, the string of data is reproduced as a noise signal having random characteristics.
In the case of packet data shown in FIG. 3, that is, in the case of a string of data including pieces of synchronization data 51, each being of a fixed pattern for data synchronization and being added to a block of voice data, the repeatedly transmitted pieces of synchronization data 51 have a constant transmission cycle (T). Further, since the blocks of voice data 52 included in the packet data are data having random characteristics, noise signals reproduced at the receiver 30 side result in noise signals including two kinds of noise signals combined therein, one being a sine wave having a frequency (f [Hz]), which is the reciprocal of a packet transmission cycle (T [sec]) as represented by the following expression: f=1/T, the other one being a noise signal having random characteristics.
A voice transmission system employing a digital modulation scheme has a disadvantage in that it is difficult for a microphone to be prevented from picking up output signals radiating from the transmitter 20, and thus, to date, in order to overcome this disadvantage, various measures have been considered.
For example, in order to shield an amplifier inside a microphone from external radio waves, a configuration, in which parts such as radio wave intrusion preventing parts are added, has been proposed. Such an addition of parts to the microphone enables prevention of intrusion of transmission signals from a transmitter into the inside of the microphone, and thus, enables prevention of transmission of voice signals including noise signals from the transmitter to a receiver. However, when a compact microphone is used, there are lots of cases where it is difficult to add the parts for shielding the microphone from external radio waves. Further, difficulty in adding parts to a microphone after commencement of its use leads to a disadvantage in that it is difficult to reduce noise signals in the case where existing microphones are used.
Furthermore, several proposals associated with a signal processing configuration for preventing the above-described intrusion of noise signals have been suggested. For example, in PCT Japanese Translation Patent Publication No. 2002-521945, a configuration, in which voice signals inputted to a transmitter are divided into a plural of frequency bands by using an FFT, and amplitude levels of signals within a frequency band including a frequency corresponding to an interference are suppressed by using a comb-type filter, is disclosed.
However, such a configuration, in which an FFT and a comb-type filter are provided, increases a processing time, and as a result, leads to a disadvantage in that a large amount of transmission delay time (more than several seconds) occurs in transmission of the voice signals. Such a transmission delay time may be acceptable, for example, in mobile wireless communication systems such as mobile telephone systems, however, it is difficult to adopt such a configuration in systems in which a large amount of transmission delay time of voice signals is not acceptable, such as wireless microphone systems.
Furthermore the configuration, in which the FET and the comb-type filter are provided, results in suppression of amplitude levels of signals having frequency components within the whole frequency band including a frequency corresponding to the interference, and thus, has a disadvantage in that the clarity of transmitted voice signals is reduced. Moreover, the configuration, in which the FET and the comb-type filter are provided, leads to an increase of load on a processor for performing signal processes, and it is difficult to avoid an increase in device cost.
Further, in Japanese Unexamined Patent Application Publication No. 2007-43555, a configuration, in which stubs for eliminating signals of high-frequency components are utilized, has been disclosed. This configuration enables elimination of signals having very high-frequency components, but has a disadvantage in that it is difficult to eliminate signals of frequency components within a low-frequency band, such as a voice-frequency band.